1. Field of the Invention
This invention relates generally to semiconductor light sources, spectroscopy, polymer waveguides and multi-wavelength laser arrays.
2. Description of Prior Art
Spectroscopy refers to the use of multi-wavelength radiation to non-invasively probe a variety of samples to determine the composition, health, or function of those samples. Prior-art spectroscopy is done with filtered white light sources, as illustrated in the prior art FIG. 1. Here, a white light source 100 emits a broadband radiation 130, which is filtered with a tunable monochromator 110, comprising a rotating grating 114 and slit 118, to generate a narrowband radiation 150, which probes a sample 120. A diffuse reflected radiation 160 is then detected by an optical detector 140. By tuning the monochromator 110, it is possible to construct a spectrum of the reflected radiation 160, which provides non-invasive information about the sample 120.
Although it enables spectral measurements over a wide wavelength range, the prior-art white light spectrometer of FIG. 1 suffers from a number of limitations. First, the filtered white light source has weak signal to noise ratio. Second, the grating-based system has critical intra-system mechanical alignments, and contains moving parts, leading to a bulky and complex system with slow measurement times. Lastly, some applications, such as (B. Tromberg, N. Shah, R. Lanning, A. Cerussi, J. Espinoza, T. Pham, L. Svaasand, and J. Butler, “Non-Invasive In Vivo Characterization of Breast Tumors Using Photon Migration Spectroscopy,” Neoplasia, vol. 2, nos. 1-2, January-April 2000, pp. 26-40) employ frequency domain measurements, which are not presently possible with white light sources, since white light sources cannot be easily modulated at the required 100 Mhz to 3 Ghz rates.
One solution to these problems is to replace the white-light source with a tunable laser. This eliminates the rotating grating 114, since the laser provides a source of tunable narrow-band radiation which requires no further filtering. However, prior art tunable semiconductor lasers, such as those described in (B. Mason, S. Lee, M. E. Heimbuch, and L. A. Coldren, “Directly Modulated Sampled Grating DBR Lasers for Long-Haul WDM Communication Systems,” IEEE Photonics Technology Letters, vol. 9, no. 5, March 1997, pp. 377-379), are limited in tuning range to less than 100 nanometers (nm), because of the fundamental gain-bandwidth limit of semiconductors. Most spectroscopic applications, such as near infrared spectroscopy from 1100-2500 nm, agricultural spectroscopy from 700-1700 nm, or tissue spectroscopy from 650-1000 nm, require several hundred nm bandwidth.
Telecom systems typically employ single-mode fibers, so tunable optical sources for telecom must be designed such that all wavelengths emerge from a common spatial location, and can be easily coupled into the single-mode fiber. In contrast, many systems employing diffuse optical spectroscopy can operate with a tunable source in which various wavelengths emerge from different spatial locations. Another way of stating this is that diffuse optical spectroscopy can use multi-mode sources, or sources emerging from multi-mode fiber. This is true if the sample undergoing spectrum analysis is spatially uniform over a distance larger than the spatial separation of sources, or if scattering within the sample causes the entrant light to spread out spatially over a distance larger than the source separation. Thus, construction of tunable sources for spectroscopy can employ designs that would not be appropriate for single-mode telecom sources. These same designs, however, could also be used in multi-mode communication systems.
Other prior art researchers, such as those in (B. Tromberg, N. Shah, R. Lanning, A. Cerussi, J. Espinoza, T. Pham, L. Svaasand, and J. Butler, “Non-Invasive In Vivo Characterization of Breast Tumors Using Photon Migration Spectroscopy,” Neoplasia, vol. 2, nos. 1-2, January-April 2000, pp. 26-40), have assembled multiple discrete lasers to access wavelengths outside the gain bandwidth limitation of a single semiconductor laser. Such an approach employing separately packaged lasers, however, introduces complexity and cost while suffering from sparse and insufficient wavelength coverage. This leads to a bulky and complex system, typically involving complex optical coupling components or multiple optical fibers.
Therefore, what is needed in the art is a compact multi-wavelength light source for spectroscopic characterization of a sample providing wide tuning range via a plurality of light sources, whose output spans a spatial dimension sufficiently small to enable diffuse reflectance or transmittance spectroscopy of a sample. This system can be used in a variety of spectroscopy applications.